1. Field of the Invention
The present invention relates generally to wave transmission apparatus, and more particularly to an improved compandor with improved signal-to-noise ratio and decreased distortion.
2. Description of the Prior Art
Signal transmission and recording apparatus have a dynamic range that is limited for low level signals by the noise in the system, and by distortion for high level signals. Since signals to be transmitted or recorded often have a larger dynamic range than that of the system, compandors, such as the one shown generally in FIG. 1 of the drawing, have been used for many years. The typical prior art compandor includes a compressor 10 which compresses the dynamic range of the input signal prior to transmission or recording, and an expandor 16 which expands the signal upon reception or playback to its original dynamic range.
A compressor typical of the prior art is shown in FIG. 2. The compressor includes a resistor 20, an operational amplifier 26, an operational transconductance amplifier (OTA) 30, a rectifier 34, a capacitor 36 and a resistor 38. The resistor 20 is connected between an input terminal 22 and the current summing terminal 24 of the operational amplifier 26, whose output is connected to an output terminal 28. The OTA has three terminals, an input, an output, and a control terminal 32. The input terminal of the OTA 30 is connected to the output terminal 28, and the output terminal is connected to the current summing terminal of the operational amplifier 24. The rectifier 34 is connected between the output terminal 28 and the OTA control terminal 32. The capacitor 36 and the resistor 38 are each connected from the OTA control terminal 32 to a ground.
In operation, the gain or the loss effected in an input signal impressed upon the input terminal 22 is a function of the gain of the operational amplifier 26. This gain is determined by the ratio of the effective resistance of the OTA 30 to the series resistance 20. The effective resistance of the OTA, which is inversely proportional to its conductance, is inversely proportional to the voltage impressed upon the control terminal 32. This control voltage is derived by rectifying the output signal on the output terminal 28. The control voltage is filtered by the capacitor 36. As the input signal level on terminal 22 increases, so will the output level at terminal 28. The increased output level, however, will decrease the amplifier gain so that the output level, although increasing, will not increase as fast as the input signal and thus, will be compressed. The combination of the capacitor 36 and the resistor 38 determine the compressor time constant. This time constant and the use of a band pass filter 40 will both be discussed later.
An expandor typical of the prior art is shown in FIG. 3. The expandor includes an OTA 42, an operational amplifier 48, a rectifier 52, a capacitor 56, a resistor 58 and a feedback resistor 60. The OTA 42 has three terminals, an input, an output, and a control terminal 54. The OTA input is connected to input terminal 44. The OTA output is connected to the current summing input terminal 46 of the operational amplifier 48 whose output is connected to an output terminal 50. The rectifier 52 is connected between the input terminal 44 and the control terminal 54 of the OTA 42. The capacitor 56 and the resistor 58 are each connected from the OTA control terminal 54 to a ground. The resistor 60 is the feedback component of the amplifier 48, connected between the amplifier input terminal 46, and the output terminal 50.
In operation, the rectifier 52 rectifies the input signal at terminal 44. The rectified signal is then filtered by the capacitor 56 and applied to the control terminal 54 of the OTA 42. The gain or the loss of the expandor is proportional to the ratio of the conductance of the OTA 42 to the conductance of the feedback resistor 60. As the input signal at terminal 44 increases, so will the control voltage at terminal 54, thereby increasing the conductance of the OTA 42 and thus the gain of the expandor. As a result of the output signal at 50 will increase more than the input signal at 44, and will thus be expanded. The combination of the capacitor 56 and the resistor 58 determine the expandor time constant. This time constant is usually made similar to the time constant of the compressor.
A major problem of prior art devices is the choice of this time constant. If a high level signal should pass through the facility, the high level signal will produce a large voltage output from the rectifiers and increase the conductance of the OTAs. This will decrease the overall gain of the compressor and increase the overall gain of the expandor. Should the time constants be made long and a large signal, as discussed above, be followed by a very low level signal, the compandor will be unable to follow the change; in other words, it will have a long "hang time." The effect is that the compressor will tend to further decrease the already low signal amplitude and thus decrease the system signal-to-noise ratio. The expandor will also be slow in following the signal change and will continue to have a high gain. This high gain will amplify the facility noise and produce what is called a "noise tail."
Conversely, if the system time constant were to be made very short, the system response will be very rapid, but the capacitors will no longer be able to fully filter the low frequency signals, and the ripple on the OTA control terminals will modulate the signal passing through the compandor. In other words, the compandor gain will flow the individual cycles of the low frequency components rather than their envelope. This is undesirable as it tends to produce distortion. Further details of the prior art compandor shown in FIGS. 2 and 3 can be found in Signetics' preliminary specifications of the NE570/571, dated February, 1975.
One prior art solution uses several compressors of the type shown in FIG. 2. Each of the compressors has an associated band pass filter 40 tuned to a different portion of the spectrum and each has a different compressor time constant; the lower frequency compressors of course having the longest time constants. The filter-compressor elements are connected in parallel. A similar parallel combination of filter-expandors, such as the expandor and the band pass filter 62 of FIG. 3, is used at the receiving end of the system. The difficulty with this approach is that in the crossover regions of the filters, the phase shifts of each of the branches do not track. Thus, a signal near the band limit will pass through two different compressor or expandor elements, each with a different phase shift or delay, and when summed at the output the components might tend to add in phase to reinforce each other or add out of phase and cancel each other. This produces distortion and signal drop outs.
Another prior art solution is disclosed in the U.S. patent to Winter, U.S. Pat. No. 3,930,208. This approach combines filter-compressor elements (and also the filter-expandor elements) in series rather than in parallel. In this arrangement the phase shifts introduced by the filters are of no consequence to the operation of the compandor. Although somewhat of an improvement over the apparatus described above, this approach still uses filters in the signal path and thus still introduces some phase shift, which may have to be compensated for in order to meet envelope delay distortion specifications for the channel.